The studies here proposed are addressed to the problem of defining physiologic mechanisms for complex coordinated movements. In previous experiments we have described the spatial and temporal characteristics of eye and neck muscles, co-activation during the horizontal rotation of the head elicited by the appearance of an unexpected stimulus in the monkey's visual field. We have called this mode of coordination "triggered" and have distinguished it from those eye-head movemens which are made in anticipation of a stimulus whose occurrence can be predicted. Our evidence has indicated that the difference between these two modes lies in the timing of neuromuscular activation. One of the main purposes of the proposed plan is to use the notion of the strategies to investigate the central neuronal mechanisms underlying eye-head coordination. To this end, we will examine those circuits which are directly impinging upon spinal cervical motor neurons as well as on the oculomotor nuclei. As a first step, we will investigate the cell discharge conveyed by two extrapyramidal pathways (reticulo- and vestibulo-spinal neck motoneurons during eye-head coordination. Our aim here will be twofold: on one hand to investigate whether the command signal conveyed by these descending pathways can be correlated with some parameter of head movement such as force, velocity, and/or position. On the other, to observe whether there is a differential involvement of these tracts with regard to the three strategies of eye-head coordination (the triggered, the predictive, and smooth pursuit). In the course of previous experiments we have shown that the coordination between the eyes and the head is a joint result of centrally initiated eye and head movements accompanied by the intervention of vestibular and neck afference. Accordingly, we propose here to define the degree of control exerted by these loops on the cell of origin of descending reticulo- and vestibulo- spinal tract and to evaluate the individual contribution of these feedbacks by assessing the short and long term behavioral consequences of interrupting these loops.